The present invention relates to an apparatus and method for improving the low speed performance of 2-cycle marine engines and, more particularly, to an apparatus and method for utilizing engine exhaust to heat the engince crankcase in an outboard motor to promote the evaporation of fuel puddles occuring therein.
2-cycle marine engines, commonly used for outboard motors, utilize the crankcase as part of the fuel supply system. A fuel-air mixture passes through a pressurized crankcase system in route to the inlet manifold and engine combustion chamber. The heavier fraction of the fuel, comprising a gasoline and oil mixture, tends to condense inside the crankcase and collect in low lying areas of the crankcase. This phenomenon is known as "puddling" and causes erratic idle and low speed operation and, when the engine is accelerated from idle, the oil-rich puddles pass through the engine resulting in undesirable smoking and poorer acceleration of the engine. Fuel puddling may also occur in other parts of the eninge fuel supply systems, such as the inlet manifold.
Another characteristic of 2-cycle outboard motors is the need to provide exhaust pressure relief at idle or low speed operation. The exhaust system in an outboard motor typically channels the engine exhaust from the exhaust manifold downwardly through the driveshaft housing and out an exhaust outlet in the propeller hub below the water level. When the engine is idling or running at very low speed, however, the exhaust passages in the driveshaft housing fill with water and block the escape of the exhaust. The resulting back pressure adversely affects engine operation, unless some form of relief is provided. In general, pressure relief is provided which allows the exhaust gases to escape upwardly through the higher internal water level in the drive shaft housing and out through an external exhaust opening in the upper portion of the driveshaft housing above the water line. U.S. Pat. Nos. 3,310,022 and 3,967,446 disclose idle exhaust pressure relief systems of this type.
Alternately, an idle relief passage may interconnect the exhaust manifold directly with the open area in the upper portion of the driveshaft housing from which it is exhausted directly to the outside through a similar opening in the housing above the water line. This type of idle exhaust pressure relief system is shown in U.S. Pat. No. 4,668,199.
The auxiliary idle exhaust or idle relief system is intended only to accommodate a relatively low volume of exhaust gas flow at idle or low engine speed. At high speeds exhaust back pressure problems are eliminated and the idle relief system not operative. It is also known to provide an idle relief system with a relatively tortuous path and/or a baffle system within the upper portion of the driveshaft housing to attenuate and muffle the exhaust noise.
Various methods and apparatus are known in the art to eliminate fuel puddling or the adverse affects thereof. A simple early solution involved draining the puddles from the low lying areas within the crankcase directly to the outside. However, potential environmental concerns have lead to other solutions of the problem. U.S. Pat. Nos. 3,800,753, 3,805,751 and 4,590,897 describe apparatus for collecting puddled fuel in the crankcase and recycling it within the engine for eventual combustion. U.S. Pat. No. 4,534,333 describes an idle speed air-fuel passage which is heated by directing the passage through the engine exhaust chest or manifold to heat the air-fuel mixture before entry into the crankcase and combustion chamber. U.S. Pat. No. 4,082,068 discloses the thermostatic control of engine cooling water to maintain a high engine temperature at idle to help prevent condensation and puddling of fuel in the crankcase. However, maintaining a higher engine block temperature only indirectly raises the temperature of the engine crankcase.
It is also known to utilize engine heat to heat the inlet manifold in 4-cycle automotive engines. However, the absence of oil in the fuel and construction of the fuel supply system obviates fuel puddling in 4-cycle engines.
It has been found that, elevated engine temperature are desirable because the higher temperature minimizes condensation of the heavier fuel-oil fractions. However, fuel puddling is not a problem at high engine speed and, in addition, lower engine temperatures result in better and more efficient high speed operation. Thus, the various prior art means for eliminating puddling described above, in addition to requiring the use of supplemental apparatus on the engine, are only needed during the limited periods of idle or low speed operation and are not needed for nor conducive to efficient operation at higher speeds.